buffer~ referencing not working on init

I’ve implemented buffer~ referencing as in Tim’s example (http://74objects.com/2009/03/25/custom-data-types-1/). However whilst this updates the reference when creating and deleting buffer~ objects, if you specify a reference to an existing buffer~ object that has not been created or deleted within the lifecycle of my object, the reference is not made. I presume this is because buffer~ is not notifying my object that it exists.